Fiber Optic Sensor Measures Sea's Saltiness

MADRID, Spain — Bathers can tell that the Dead Sea is extremely salty because all they can do in it is float. Physicists at the Universidad Complutense say they have a more precise way to measure maritime salinity: a fiber optic sensor.

Although the researchers have so far tested it only in their laboratory, preliminary results indicate that it has great potential for the real-time monitoring of both sea salt and contamination levels.

The sensor will be simpler and less expensive than the traditional method of checking salinity with an electrical conductivity meter, the researchers say. Fiber optics also enable better access to hard-to-reach places and integration into complex monitoring systems. However, the technique is far from becoming a commercial reality. It is part of a three-year European Union research program aimed at developing an integrated system of fiber optic sensors to monitor a variety of physical and chemical parameters in the oceans.

A fiber optic sensor could provide real-time monitoring of the saltiness of the sea.The Spanish team based its technique on water's refractive index. At a constant temperature, the index varies linearly with changes in salt concentration. The researchers compared the sensor's refractive index values with values predicted by theory and found their numbers were on the mark.

Described in the Sept. 1, 1999, issue of Applied Optics, the experimental setup was based on a side-polished single-mode optical fiber, on which was deposited a transducing element composed of two thin layers: the first of aluminum and the other of titanium dioxide.

The researchers fitted an 807-nm light-emitting diode with a polarizer and guided the light through the fiber, exciting plasmons on the surface of the aluminum and thus attenuating the power transmitted through the fiber.

This attenuation is strongly dependent on the refractive index of the external medium, in this case water. So, by carefully choosing the parameters of the two layers, the researchers can fine-tune the response of the transducing element to changes in salt concentration and make the relationship linear. Their best results came with an 8-nm layer of aluminum coated with 56 nm of titanium dioxide.